Ending "trafficking" is perhaps the most well-known, well-resourced, well-loved social cause of the 21st century that doesn't require its proponents' agreement on what it even is they wish to end. What is "trafficking"? How many people are "trafficked"? Look beyond the surface of the fight against trafficking, and you will find misleading statistics and decades of debate over laws and protocols. As for the issue itself, the lack of agreement on how to define "trafficking" hasn't slowed campaigners' fight. Rather, defining trafficking has become their fight.

Over the last 15 years, anti-trafficking campaigns have ascended to the most visible ranks of feminist, faith and human rights missions, enjoying support from organizations as ideologically unneighborly as women's rights NGO Equality Now and the conservative thinktank the Heritage Foundation. The dominant contemporary understanding of trafficking took hold only around the turn of the century, driven in significant part by the advocacy of women's rights groups who sought to redefine trafficking specifically as the "sexual exploitation" of women and children. Indeed, this is the definition that groups like the Coalition Against Trafficking in Women succeeded in getting written into the first international laws (pdf) related to trafficking. When many people hear about "trafficking", this is the picture that comes to mind, and it is one that women's rights groups – and, increasingly, evangelical Christian groups like Shared Hope International, International Justice Mission and Love146 – use explicitly in their campaigning.

In fact, it is with the influence of these groups that the issue of trafficking itself began to migrate: from relative obscurity, to sessions at the United Nations, to highways in the United States, where drivers now find anti-trafficking billboards featuring shadowy photographs of young girls, images meant to "raise awareness". Using such images in service of ending "sexual exploitation" may seem contradictory, as the girls photographed are presented in what would otherwise seem to be a quasi-pornographic setting. A billboard I passed by last August on the interstate in Louisiana showed a girl who looked sweaty and wide-eyed – only, with the words "'NOT FOR SALE: END HUMAN TRAFFICKING" stamped in red over her face.

But what – and who – is being fought in what is pictured here?

Accurate statistics on trafficking are difficult to come by, which does not stop some anti-trafficking groups from using them anyway. For instance, Shared Hope International, which is aggressively pursuing anti-trafficking legislation in 41 US states, claims "at least 100,000 juveniles are victimized" each year in the United States, and possibly as many as 300,000 – a figure that has been cited (repeatedly) by CNN. In truth, the figure is an estimate from a University of Pennsylvania report from 2001 (pdf) of how many youth are "at-risk" of what its authors call "commercial sexual exploitation of a child", based on incidences of youth homelessness. But it was not a count of how many youth are victims of "trafficking'", or involved in the sex trade.

Prostitution is often conflated with "trafficking" in these statistics, in part because the definition of trafficking that has been pushed to prominence refers exclusively to "sexual exploitation". In fact, this conflation has found its way into the collection of data: according to a report from the Global Alliance Against Trafficking in Women:

"[W]hen statistics on trafficking are available, they usually refer to the number of migrant or domestic sex workers, rather than cases of trafficking."

This purposeful conflation of sex work and trafficking distinguishes the many feminist and faith-based anti-trafficking groups that focus on sex-trafficking from groups that work directly with people who are involved in forced labor of all kinds, whether or not it involves sex work. This schism over who gets to define trafficking is about to come to a head for California voters in the form of Proposition 35, which, if passed this November, will set higher criminal penalties and fines for those who commit what the authors of the bill define as sex-trafficking, as opposed to labor trafficking. It will also force those convicted of "trafficking" to register as sex offenders and submit to lifelong internet monitoring – whether or not sex or the internet were involved in their case.

There is no question that California is home to several industries where, according to the Freedom Network (whose member organizations employ a human rights-based approach to anti-trafficking work), forced labor is known to occur: agricultural work, restaurant work, construction work, hotel work, garment work and sex work. But what unites the workers in these industries is not "commercial sexual exploitation", but the erosion of labor protections and the toughening-up of immigration policies – which makes many more people vulnerable to coercion and abuse. Cindy Liou, staff attorney at the Asian Pacific Islander Legal Outreach, a Northern California-based project that has worked with hundreds of survivors of human trafficking, is advocating for Californians to vote no on this proposition.

"It redefines trafficking in a way that's incorrect, confusing, and terrible. It's highly problematic because it does this at the expense of all trafficking victims – it ignores labor trafficking victims."

In fact, so-called "sex-trafficking" constitutes only a small percentage of all forced labor, as estimated by the International Labor Organization. In their 2012 report, the ILO categorizes "forced sexual exploitation" as distinct from other kinds of forced labor, which, in some ways, is helpful as not all forced labor involves sex, and in other ways, adds more confusion to the issue (because what distinguishes "sex-trafficking" from sexual abuse is its occurrence within a workplace or work relationship). Still, of the nearly 21 million people the ILO estimates are forced laborers, 4.5 million are estimated to be what the ILO calls "victims of forced sexual exploitation" – that is, over three-quarters of the people around the globe estimated to be in forced labor are not involved in "forced sexual exploitation". By contrast, the ILO estimates that 2.2 million people worldwide are forced laborers in prisons or in the military – people whose stories we are quite unlikely to find on a highway billboard.

But, as in the furor over "sex-trafficking", it would be wrong to let the volume of public outcry stand in for how significant a problem it is – however that issue is defined or ill-defined. Similarly, the push for "real numbers" can also lead us further into the ideological morass that itself defines the trafficking issue. Jo Doezema, a researcher with the Paulo Longo Research Initiative and author of Sex Slaves and Discourse Masters: The Construction of Trafficking, cautions:

"[E]ven a recognition that disputes over the meaning of trafficking involve politics and ideology does not go far enough: it still leaves intact the idea that trafficking can be defined satisfactorily, if political will, clear thinking, and practicality prevail."

That is, there can be no assessment of the severity of "trafficking" if we define this issue by a simple and coherent accounting of "victims". What's lost in the relentless defining and counting are the complex factors behind what is now almost unquestioningly called "trafficking". Most of all, what is lost is any understanding or appreciation of the challenges faced by the millions of people working, struggling and surviving in abusive conditions, whose experiences will never fit on a billboard.

• Editor's note: the photo caption originally stated that the ILO survey referred to trafficking victims; in fact, it refers to victims of forced labor, which may include trafficking victims, but it did not specifically attempt to count those. The caption was amended at 1pm (ET) on 24 October